1. Technical Field
The invention relates generally to startup burners and specifically to startup burners used in chemical recovery boilers in the pulp and paper industry.
2. Related Art
Chemical recovery boilers isolate useful compounds from manufacturing byproducts. In the pulp and paper industry, pulp mills typically use a manufacturing process in which wood chips or other lignocellulosic biomass are treated with chemical liquor comprising cooking chemicals. The wood chips or other lignocellulosic materials are then cooked in a digester at predetermined temperature and pressure to form a slurry comprising spent liquor and a rough pulp with inconsistent particle size. After cooking, equipment washes the spent chemical liquor from the rough pulp. The spent liquor is commonly known as “black liquor” and comprises organic and inorganic chemicals left over from the cooking process. The pulp is generally sent to other equipment for further refinement. The black liquor is eventually pumped to a chemical recovery boiler and processed to recover the cooking chemicals. Without recovering and reusing the cooking chemicals from the black liquor, the cost of industrial paper-making processes would be prohibitive.
Chemical recovery boilers generally evaporate excess moisture from black liquor solids, burn organic liquor components, supply heat for steam generation, and recover inorganic compounds—notably sodium sulfide and sodium carbonate. Some of these compounds can be re-causticized and used elsewhere in the manufacturing process.
In the recovery process, the black liquor is typically concentrated into a solution containing a solids concentration of above sixty percent by mass. Nozzles in the furnace wall then spray black liquor into a furnace. The nozzles are generally located in the bottom quarter of the furnace and may be several meters above the bottom of the furnace. The furnace is a reactor that generally dries and partially pyrolyzes the liquor droplets as they fall toward the bottom of the furnace. The furnace also evaporates, gasifies, oxidizes, and reduces, components within the black liquor to recover the cooking chemicals.
The partially dried and reacted black liquor accumulates in a mound at the bottom of the furnace known as a “char bed”. Nozzles typically permit airflow into the furnace at a low, middle, and upper elevation. The air, together with the lignin, wood extracts, and other organic compounds maintain combustion in the furnace. Inorganic compounds are often reduced in the char bed into a molten smelt. The smelt may accumulate and flow out of the furnace through a smelt spout and into a collection tank. These reactions consume heat. As such, operators generally regulate and redistribute airflow and black liquor input, to promote and maintain combustion for efficient chemical recovery.
In traditional recovery boilers, the furnace is internally lined with a series of densely-arranged, high-pressure coolant-filled tubes. The coolant is commonly water and a collective series of tubes is generally known as a “water wall.” To regulate temperature efficiently, the water wall tends to cover a large internal surface area. In some existing chemical recovery boilers, three inch coolant tubes are generally separated by one inch filler bars so as to form a gas-tight barrier enclosing the furnace.
To operate safely and efficiently, the furnace generally operates under negative pressure. A constant inflow of air near the base of the furnace is generally required to maintain combustion and to replace air and other gases that exit the recovery boiler near the top of the furnace. Air generally enters the otherwise gas-tight furnace through openings in the furnace water walls. Such openings include air ports and throats, which are designed to inject pressurized air. Ambient air generally flows through other openings, such as those for smelt spouts, due to the negative pressure in the furnace. For most such openings, the coolant tubes generally bend around the opening in the furnace wall.
Air manifolds or windboxes generally flank the throat and air port openings on the outer wall of the furnace. Large fans ducted to the windboxes can cause air to flow into the furnace through the various throats and air ports in the furnace walls.
Airflow is the primary variable of operation aside from the rate of black liquor input. Large quantities of air are generally forced through the narrow throat and air port openings to maintain combustion. The flow of air through a throat and, diffuser, or swirler is desirable to maintain auxiliary combustion from active startup burners. Unfortunately, conditions within the furnace contribute to the gradual obstruction of air flow as smelt slowly accumulates over the various openings. Over time, accumulations of frozen smelt on and around the coolant tubes can grow to obstruct the openings, thereby reducing an operator's ability to regulate combustion. Recovery boilers may need to be deactivated when smelt accumulations significantly interfere operation. This extensive maintenance period results in loss of production.
Temperature is another variable of operation. Startup burners help regulate internal furnace temperature. Startup burners are auxiliary burners that commonly fire natural gas, propane, and/or fuel oil, and are generally used to initiate combustion within the furnace after a period of dormancy. Once the startup burners increase furnace temperature to an established minimum, liquor firing can commence. Liquor firing is then increased until the liquor itself sustains combustion. The startup burners are then generally deactivated. Startup burns have also been used to provide supplementary heat to the furnace when liquor flow is interrupted or insufficient to meet boiler demand.
When inactive, the startup burner generally rests in the windbox within a burner housing adjacent to the throat opening. Radiant heat from the furnace can damage inactive startup burners. Moreover, splashes of black liquor through the throat openings can cause smelt fouling directly on the startup burner, particularly on the firing end of the startup burner, comprising, for example, the fuel nozzles, swirler, igniter assembly, and flame detection equipment. Smelt fouling can render the startup burner ineffective, unsafe, and unreliable.
There is a need to increase the intervals between recovery boiler maintenance and to reduce the amount of maintenance time while preserving or improving the operability of the recovery boiler after said maintenance.